The intense vasoconstriction that occurs in the renal vascular bed as a result of increased activity of the sympathetic nervous system can be caused by two sympathetic mechanisms in the dog: 1) Activation of the renal sympathetic nerves and 2) Adrenal catecholamines reaching the kidney via an adreno-renal vascular network. Both of these sympathetic mechanisms should facilitate renal retention of sodium. The first objective of this proposal is to measure and compare the changes in intrarenal cortical blood flow, glomerular filtration and sodium excretion mediated by these sympathetic mechanisms. Experimentally-induced cardiac arrhythmias will be used to elicit these sympathetic mechanisms in the pentobarbital anesthetized dog because with the onset of a tachyarrhythmia there is an abrupt decrease in mean arterial pressure and cardiac output which results in an immediate compensatory increase in sympathetic tone (activation of the renal sympathetic nerves). This activation of the renal sympathetic nerves causes a disproportionately greater reduction in renal blood flow during the hypotension than can be accounted for by the diminished cardiac output. Upon cessation of an arrhythmia there is rapid return to control levels of flow in all vascular beds except the renal. We found that this post-arrhythmic renal vasoconstriction is due to catecholamines which reach the kidney from the adrenal medulla without entering the systemic circulation via an adreno-renal vascular network. The next objective is to determine the mechanism which causes adrenal catecholamines to flow through the adreno-renal vascular network to produce renal vasoconstriction and whether this mechanism is activated by cardiovascular responses not related to cardiac arrhythmias. The final objective is to investigate the importance of these sympathetic mechanisms in the renal retention of sodium during the development of DOCA-salt hypertension in the rat and congestive heart failure in the dog (animal models in which there is increased activity of the sympathetic nervous system).